I'm a tax preparer in Texas.
I have never seen federal taxes withheld, but often state taxes are, when winnings are over $1200. (That might not be a magic number, it just always seems to be that amount or higher.) So in most cases it would be the state and not the IRS that you would have to deal with.
I've been down here for 14 years so it has been a while since I have done a Canadian return. But there is a foreign tax credit which may apply to casino winnings.
I found this reference, which may help:
http://www.taxtips.ca/filing/foreigntaxcredit.htmI have not read this exhaustively, but it applies to non-business taxes. It doesn't exclude casino winnings, so one would think that the credit would apply.
Cheers. .../don